Disaster recovery techniques that can tolerate the failure of a complete data center have become increasingly important. A classical technique used for tolerating data center failures is copying the data to a remote site. This technique is known as remote-mirroring. There are various forms of remote-mirroring. They can primarily be classified as either synchronous or asynchronous. Synchronous remote-mirroring techniques delay acknowledging an I/O to a host until it has been executed on the remote site as well. Asynchronous techniques, on the other hand, acknowledge an I/O as soon as it executes on the primary site.
Synchronous remote-mirroring techniques usually have high impact on I/O performance, high bandwidth requirements, zero data loss, and very small time to recovery. Asynchronous remote-mirroring techniques usually have low or no impact on I/O performance, low bandwidth requirements, possibility of data loss, and higher time to recovery.
Asynchronous mirroring techniques can be classified further along two dimensions, including write-ordering and write-coalescing. Write-ordering is focused on maintaining the order of writes at the remote mirror. Write-coalescing is focused on coalescing multiple writes to the same block (at the mirror) into one write. Write coalescing reduces bandwidth requirements when write-locality exists.
An asynchronous technique that provide write-coalescing to reduce bandwidth requirement, and also achieve most of the benefits associated with synchronous mirroring, are desirable.